A wide variety of different viruses are responsible for causing viral infections of varying severity in animals and man. Because of the large number of potentially serious or widespread epidemics, e.g., influenza, resulting from viral infections, methods are constantly being sought for either prevention or cure of the diseases caused by these entities. The difficulty in finding cures for diseases caused by entities about which so little is understood has tended to lead scientists to concentrate on the preventive aspects of viral disease. Along these lines, development of vaccines which can prevent or lessen the symptoms of viral infection has long been a goal of immunologists. Two principal types of vaccine are currently enjoying the most widespread use. One type is the attenuated live virus type of vaccine in which the virus has been rendered avirulent but not killed, either through a serial passage through a number of host cells, or, less commonly, by some form of genetic attenuation. While this type of vaccine has achieved a measure of success, the primary drawback is that the attenuated virus may revert back to its virulent state while living in the host's body. Another type of vaccine consists of virus particles unactivated by formalin or other chemical or physical treatment. The principle behind the use of the killed viruses is that, in theory, the viral antigens which are responsible for producing the required protective immune response remain intact while the ability of the virus to replicate in the host has been destroyed by the formalin treatment. This formalin-inactivated type of vaccine is probably the most common on the market today; however, despite the widespread use of this technique of vaccine preparation the resulting products are really not very effective because the formalin treatment often chemically alters the antigen in some way so as to render it less potent in eliciting an immune response in the host.
In an attempt to circumvent the problems encountered with the two commonly used vaccine types, considerable effort has been put into attempting to isolate the antigenic components of the viral coat. Theoretically, the isolated antigens would be capable of stimulating the host's immune system even in the absence of the other parts of the virus; at the same time, there would be no live virus present, and hence no possibility of infection. The viral antigens must, of course, be isolated in a manner which does not alter them chemically, and allows the antigenicity to be fully retained. Some of the most useful components in this regard are the glycoproteins which make up part of the viral envelope. Glycoproteins are found as structural and/or functional elements of virtually all viruses, and are frequently highly antigenic. A number of glycoprotein preparations derived from viral envelopes have been suggested as possible active components for vaccine compositions. For example, U.S. Pat. No. 4,470,967 describes vaccine preparations which are made by complexing viral glycoprotein with a lectin, the latter element acting as an adjuvant. A number of references, e.g., as described in U.S. Pat. Nos. 4,344,935, or 4,356,169 or Morein et al., J. Gen. Virol. 64: 1557-1569, 1983, utilizes a method of preparation of paranfluenza glycoprotein compositions in which the viral glycoprotein HN and F are solubilized with a detergent, to extract them from the viral envelope, followed by some method of phase separation in order to remove the detergent and lipids. The latter procedures claim to produce a glycoprotein subunit which is not only substantially detergent free, but also lipid free. The latter type of highly purified glycoprotein is currently touted as the preferred type of active agent for potential use as commercial vaccine. However, at present no vaccine composition containing such a purified glycoprotein preparation has been made commercially available. To a large extent this may be due to the difficulty in preparation of the glycoprotein subunit: removal of the detergent and lipid is usually achieved by separation on a sucrose gradient, a tedious procedure which is dfficult to accomplish on a scale large enough to make commercial production feasible.
It has now been unexpectedly discovered that a new method of preparation of viral subunit vaccines produces an antigenic product which is capable of eliciting an antibody response in the inoculated individual which is far superior to that obtained with the commonly used formalin-inactivated viral vaccine preparations. Furthermore, it is also simpler to prepare the isolated subunit of the present invention, since, rather than consisting of a purified, lipid-free, protein micelle as a number of the previous vaccines do, the present preparation is composed of a glycoprotein-lipid complex vesicle, which has the advantage of not requiring the removal of the normally present lipid from the solubilized glycoprotein. The fact that a glycoprotein-lipid complex could show such exceptional ability to confer immunity is particularly surprising, since conventional wisdom teaches that lipids are non-antigenic and thus their presence in a vaccine composition would be thought to reduce the efficiency, or even interfere significantly, with the ability of the vaccine composition as a whole to induce immunity. The present subunit vaccine active component is prepared, as are many of the known subunit compositions, by detergent extraction; however, a major distinction exists in that the detergent used in the extraction process is a dialyzable detergent. The specific use of a dialyzable detergent allows the relatively simple purification even on a large scale, by dialysis, of the glycoprotein-lipid complex of the present invention. Thus, the present preparations not only have the advantage of being easier to prepare than the proposed glycoprotein vaccine compositions in the literature, but also are far more effective than the currently available commercial products.